It is always desirable for display devices to reproduce easily identifiable (visible) information. It is especially important to provide high visibility of information displayed in public places (e.g., on traffic control signs).
Known methods of improving visibility, e.g., of traffic control signs, consists of increasing contrast of images by using two different colors, e.g., painting a sign with a red coating on a white background, or painting a sign with a white coating on a green background. Methods of improving visibility introduced into practice in the recent years are based on increasing contrast by using not only a difference in colors but also reflecting properties, e.g., by using colored resins that reflect headlight of an automobile.
Furthermore, in the recent years light-emitting diodes (LEDs) find rapidly growing indoor and outdoor application in publicly used display devices, and display devices that incorporate light-emitting diodes provide excellent visibility without the use of color-reflecting coatings since images can be reproduced by the light-emitting diodes themselves.
Typically, a display that uses light-emitting diodes comprises an electronic circuit board with a plurality of light-emitting diodes arranged in a matrix form, wherein images are reproduced by combining light-emitting and non-light-emitting regions, if necessary, with an addition of combinations of various colors. It is also a common practice to protect the regions illuminated by means of light-emitting diodes by coating these regions with an organic resin such as a transparent epoxy resin.
The LEDs themselves, as well as of the electronic circuit boards that support the light-emitting diodes, except for those regions that are protected by organic resin coatings, need to be protected from external environmental affects, such as water, dust, etc. Various methods of protection can be used for this purpose, but a most widely used method is coating of the LED-supporting circuit board, except for the aforementioned light-emitting regions, with a soft material, i.e., with a potting material. Since on the circuit board the potting material itself does not emit light, it constitutes a background area (non-light-emitting regions) between the light-emitting diodes.
In order to be adhesive to light-emitting diodes and the electronic circuit board, and at the same time to absorb physical deformations caused by heat and impacts, the potting material should be an elastomer. Since in a display that contains light-emitting diodes the latter should be distributed with high density, it is desirable to use such a potting material that, prior to curing, can easily penetrate into narrow spaces between the light-emitting diodes and that possesses low viscosity. In particular, if the display device is intended for outdoor application and if water penetrates into the aforementioned cracks or areas of disconnection, a potting material that has low weather-proof properties will be subject either to cracking, or to decrease in adhesive bond with light-emitting diodes and disconnection from them, or to deterioration of the circuit board. For the above reasons, an elastomer-type potting material, especially a silicone elastomer-type potting material that is capable of overcoming the above problems is the most widely used.
However, a problem associated with display devices that have background areas between the light-emitting diodes made from a potting material is that glossiness of the aforementioned background impairs visibility of the displayed image. Visibility of the displayed images is particularly impaired by light reflected from the background surfaces when a display device is used outdoors and is irradiated with solar rays. An elastomer-type potting material, in particular one that has low viscosity prior to curing, is subject to increase in glossiness after curing, but since this tendency is especially strong in silicone elastomer-type potting materials, a strong demand exists in finding a way of improving visibility by eliminating or diminishing light reflected from the backgrounds of the aforementioned type.
Heretofore, several methods were proposed for eliminating or diminishing reflection from background areas of displays.
For example, Japanese Unexamined Patent Application Publication (hereinafter referred to as “Kokai”) 2000-136275 discloses a method in which a potting material that contains a polyisobutylene-type polymer, a curing agent with hydrosilyl groups (Si—H groups), and an organic compound with alkenyl or alkynyl groups is combined with silica or a similar gloss-reducing agent. However, as can be seen from Application Example 2 of the above publication, the addition of a gloss-reducing agent could produce the value of glossiness not exceeding 45%, which is far from the gloss properties needed for the aforementioned displays.
Furthermore, Kokai 2000-136275 also discloses a method according to which glossiness on the surface obtained after curing is reduced by physically modifying the surface, e.g., by treating it with sand paper. However, if this method is used for treating surfaces of displays with a high density of distribution of light-emitting diodes, sand paper can either damage the aforementioned light-emitting diodes, or, if the elements projects from the surface of the potting material, the areas around the projecting elements become inaccessible for treatment with sand paper. Therefore, the above method did not find practical application.
Kokai H05-152606 discloses a construction which consists of a first resin layer, the main purpose of which is to secure light-emitting diodes on the electric circuit board, and a second resin layer, applied onto the first layer, the main purpose of which is to eliminate reflection. Such a construction, however, involves two resin layers on the electronic circuit board, cannot eliminate reflection in one treatment step, and requires complicated multiple-step operations.
It was further proposed in Kokai H05-152606, to impart to the surface of the first layer a reflection elimination function, instead of the formation of the second layer on the first layer. For example, as shown in FIG. 5 of the aforementioned patent publication, it is recommended to apply onto the first layer a tape-like cloth and to press this cloth to the first layer for imprinting the cloth texture on the surface of the resin layer in order to form on this surface fine unevenness, while the first layer is still in a soft state prior to complete curing.
However, the above method can be realized only at an experimental level, and is not suitable for industrial conditions. The first reason is that it is rather difficult to fix the time for transfer of the cloth texture to the resin coating during curing of the first layer. If the selected time is too short, the cloth will adhere to the first layer and it will be difficult to peel it off from the layer, and if the time is too long, it will be impossible to transfer the texture image to the layer. The second reason is that it is difficult to press the cloth to the areas in the vicinity of light-emitting diodes that project from the first layer, whereby it is impossible to provide uniform distribution of pressure over the entire surface of the first layer. As a result, the background will inevitably acquire non-uniform glossiness.
Furthermore, as shown in FIG. 6 of Kokai H05-152606, it was also suggested to apply finely powdered black resin onto the first layer, and then to remove the fine particles distributed over the surface. However, this method requires spreading of particles when the first layer is still in a soft state. Another problem is that the fine black powder is widely spread into the environment.
As shown in FIG. 7 of aforementioned Kokai H05-152606, it was further suggested to form the second layer from a mixture of fine particles of glass and silica. However, similar to Application Example 2 of Kokai 2000-136275, this method does not eliminate reflection and does not decrease glossiness to a practically acceptable level.
Thus, several proposals were offered heretofore for eliminating reflection from the background or for reducing background glossiness, but none of these proposals could reach the level suitable for practical use.
Keeping in mind the problems of the prior-art technique, it is an object of the present invention to provide a display device having light-emitting regions and non-light-emitting regions, wherein practically acceptable elimination of reflection or minimization of glossiness of non-light-emitting regions can be achieved in one reliable and easily executable treatment operation that results in obtaining good visibility of images reproduced by the display device, which is especially suitable for outdoor application.